I have the Canon 70-200 L f4 and its a great lens, pretty light so useful for many applications. It suffers a for wildlife. I don't know if I should buy a 1.4 x convertor for it (the 2x puts it into manual focus) or Chop it in against the 100 - 400 L which I have read lots of good things about.
Has anyone used the 100 - 400 L ? I am going to Shetlands next year to photograph seals and hopefully Killer Whale. I feel the 200 might not be enough.
I use the 70-200 more than the 100-400 but I have a bigger prime telephoto for the longer end so use that more.
The 100-400 is a great lens, like all long zooms it suffers with image quality right at the 400mm lens and can be quite soft. The image stabiliser is not all it's cracked up to be and you still need quite fast shutter speeds to get the best from it, especially above 300mm.
It's a push-pull zoom rather than twist like most lenses, some people blame internal dust problems on this but I can't say I've noticed any problems with it.
Even with IS I think you'll need more light with the 100-400 than the 70-200 as it's 5.6 at the longer end.
If I could have one lens only I'd take the 2.8 70-200 above both your choices, if it was a choice between the f4 and the 100-400 I'd probably go for the 100-400.
Yeah i think Hiring a Lens would be a better alternative, As OB said, hire a fast long Prime and buy the 1.4 conv, meaning youll have a nice long Lens on a Crop sensor and Still being Just as/faster than the 100-400 and the image Qualtiy would be better.
One mans rubbish is anothers treasure.....